Surface acoustic wave (SAW) devices (also referred to as “SAW filters”) are used for many applications, such as radio frequency (RF) filters and duplexers. SAW devices use the propagation of acoustic waves at the surface of a piezoelectric substrate, where their frequency is proportional to a velocity of the surface acoustic waves. SAW devices may combine low insertion loss with relatively good rejection, and can achieve broad bandwidths. Most SAW devices are sensitive to changes in temperature. However, many apparatuses and systems implementing SAW devices require the SAW devices to operate in relatively wide temperature range, such as −30 degrees Celsius to 85 degrees Celsius (deg. C.) or wider.
When the temperature of a SAW device changes, the velocity of the surface acoustic wave may also change, which may cause the SAW device response to experience a shift in frequency. This limitation has become more significant as consumer radio-frequency (RF) devices are specified to operate across a wide temperature range. Additionally, due to thermal expansion, the component dimensions of the SAW device may change, which may also lead to a frequency or response shift. These frequency shifts may result in an overall degradation of performance.
The thermal sensitivity of a SAW device is usually measured by a coefficient called the temperature coefficient of frequency (TCF), which is measured in parts per million per degree Celsius (ppm/deg. C.). Most materials used in the construction of SAW devices have a negative TCF, which typically causes a SAW device's response to shift toward lower frequencies as the temperature of the SAW device increases. However, some dielectric materials, such as silicon oxide materials, have been shown to exhibit a positive TCF. A positive TCF typically causes a SAW device's response to shift toward larger frequencies as the SAW device's temperature increases. Thus, one approach known to reduce the thermal sensitivity of SAW devices consists of burying or otherwise covering the electrodes of a SAW device under a film of silicon oxide material. By adding a silicon oxide material on top of the piezoelectric substrate, temperature compensation may be obtained, thereby allowing a SAW device to have better thermal stability.
Unfortunately, due to the properties of the piezoelectric material used in many SAW devices, the frequency shift of a SAW device response due to a change in temperature may not be uniform with the frequency of the input signal. In other words, the effect of the temperature on the frequency response of a SAW device may be a frequency shift, which may depend on the frequency of the input signal. This may lead to some deformation of the filter response. A typical SAW filter may exhibit a transition from the cutoff frequency to the pass band at the low frequency side of the graph (also referred to as a “low frequency transition”, “low band transition”, “low frequency skirt”, and/or the like) that is stable as the temperature changes. However, the transition from the passband to the cutoff frequency at the high frequency side of the graph (also referred to as a “high frequency transition”, “high band transition”, “high frequency skirt”, and/or the like) may not be fully temperature compensated even though such a SAW filter has very small TCF for the low frequency skirt. Reciprocally, the high frequency side transition may be relatively stable with the temperature while the low frequency transition shifts. This may be due to the SAW filter having a better temperature compensation for some frequencies than others.
Furthermore, some SAW filter applications may only require a relatively steep transition for one side of the response while not requiring a relatively steep transition for the other side of the response. For example, some SAW filter applications may require a 2 MHz transition between a passband and a stopband for a low frequency side of a response, while requiring a 20 MHz transition for the high frequency side of the response. However, providing relatively good temperature compensation may require forming a SAW device to have a relatively large dielectric material thickness, which may result in a reduction of the piezoelectric coupling coefficient and may limit the possible relative bandwidth of the SAW filter. Accordingly, having a small TCF only on the low frequency side of the response, for example, may allow a SAW device to meet low frequency skirt requirements while allowing a wider passband. Therefore, providing different TCFs for different portions of SAW devices may be desirable.